This invention relates to a gaseous fueled engine and more particularly to an improved method of operating a gaseous fueled engine having a feedback control system for maintaining the desired optimum fuel-air ratio under a wide variety of conditions.
In the interest of conserving natural resources and preserving the ecology, the use of alternate fuels for internal combustion engines are being investigated. Gaseous fuels such as propane, butane, and the like (commonly referred to as LPG) have been considered as optimum sources. In addition to offering greater emission control and conserving natural resources, these fuels have been found in some instances to offer longer engine life and extended service intervals.
One form of gaseous fueled engine that has proven to be quite efficient employs a charge former in the form of an air-valve type of carburetor which is fed with gaseous fuel from a source where the fuel is stored under high pressure as a liquid or partially as a liquid. The air valve carburetor tends to maintain the desired air-fuel ratio under widely varying engine conditions. However, in order to assure more effective air-fuel ratio control, a system is provided that includes an air bleed that mixes atmospheric air with the fuel supplied to the charge former so as to maintain the desired fuel-air ratio. A sensor such as an oxygen sensor is placed in the exhaust system, and the amount of air bleed is controlled by a feedback system.
An example of an effective system of this type may be found in the United States Letters Patent, entitled "Fuel Control and Fuel Feed System for Gas-Fueled Engine," U.S. Pat. No 5,337,722, issued Aug. 16, 1994, and assigned to the Assignee hereof.
Although feedback control is a very effective control for such engines, there are some conditions of engine operation wherein feedback control may not be satisfactory or the best method of control. Under such conditions it is possible to control the fuel-air ratio by memorizing maps in the ECU or CPU for the system, which maps will be indicative of the desired fuel-air ratio for the various conditions. However, the use of such additional maps and the fact that the system may also require a separate map for each type of fuel that may be used can put quite a burden on the memory capacity of the ECU.
For example, it is frequently the practice to incorporate a catalytic exhaust treatment system in the engine. These catalytic systems are normally positioned in the exhaust manifold or exhaust system, and to be operative must be maintained at a relatively high temperature. However, there is also a risk that the conditions of operation of the engine may be such that the catalyst may become overheated. The use of feedback control under such overheating conditions can, in fact, at times cause the problem to be aggravated, and thus it is necessary to utilize a system other than feedback control. If this control is done by a memory based upon certain conditions, then the aforenoted problems with memory capacity can be encountered.
Another condition where feedback control is not necessarily desirable is during starting, particularly cold starting. Frequently an arrangement is provided for enriching the mixture during cold starting. This thus requires added memory capacity for determining the amount of fuel for this respective cold-starting condition. Alternatively, rather than employing the air bleed for the enrichment, many systems include separate cold-starting enrichment circuits. This obviously increases the cost and complexity of the system.
Another running condition when feedback control may not be desirable or necessary is during extreme decelerations. It is known that under extreme decelerations the throttle valve is closed and the induction system vacuum is high. Thus, a very rich amount of fuel may be drawn into the engine on such deceleration conditions, since the engine actually operates like a pump under this condition. It has, therefore, been proposed to provide a fuel shutoff, which will shut off the fuel on deceleration. This however adds to the cost of and further complicates the system.
If, however, feedback control is also employed with a shut off deceleration valve, then the feedback control will tend to try to enrich the mixture when the fuel is shut off because the exhaust sensor will sense a lean mixture. This attempt at enrichment will cause a shutting down of the air bleed, and upon reinitiation of normal operation, the mixture will be too rich and problems can be encountered.
It is, therefore, a principal object of this invention to provide an improved gaseous fueled engine and control system therefor that employs feedback control for the primary control but which offers other forms of control not requiring memorized values in the event certain abnormal running conditions are encountered.
It is a further object of this invention to provide a feedback control system for a gaseous fueled internal combustion engine wherein the feedback control is discontinued under certain nonstandard or abnormal conditions and is resumed in such a way as to accommodate the return to normal running from the abnormal condition.
It is a further object of this invention to provide an improved air-fuel control arrangement for a gaseous fueled engine to accommodate the situation of catalyst overheating.
It is a still further object of this invention to provide an improved air-fuel control and method for a gaseous fueled engine for accommodating the start-up and idle operation.
It is a still further object of this invention to provide an improved air-fuel control arrangement for a gaseous fueled engine to accommodate normal feedback control, but a different form of control under decel conditions and without a shut off valve.